Lesson 3: Landfill Gas Movement, Control and Energy Recovery

Philip O'Leary & Patrick Walsh | Mar 01, 2002

This is the third lesson in the independent learning correspondence course on municipal solid waste (MSW) landfills. One lesson in this 12-part series will be published in Waste Age magazine each month throughout the year.

If you are interested in taking the course for two continuing education credits (CEUs), send a check (payable to the University of Wisconsin for $149) to Phil O''''Leary, Department of Engineering Professional Development, University of Wisconsin, 432 N. Lake Street, Madison, WI 53706. For more information, contact: Phil O''''Leary at (608) 262-0493. E-mail:[email protected]. Website:www.wasteage.com.

Modern engineered landfills are designed to be secure locations that protect public health and the environment from the polluting characteristics of solid and hazardous waste. Early landfill designs focused on reducing landfill nuisances, such as odors, vermin or open burning. Subsequent facility designs focused on protecting water quality. Achieving these goals, however, resulted in the containment of gases produced during waste decomposition.

Previously, most gases generated from decomposition escaped into the environment because no impervious cover existed to hold them in.

Gas composition primarily is controlled by microbial reactions from products in the landfill. Generally, a landfill goes through three stages — aerobic, facultative and anaerobic, then anaerobic — with different bacterial types dominating each stage. As gas builds up and pressure increases, it seeks ways to move upward and outward. This outward movement is cause for concern because landfill gas (LFG) constituents can be explosive and may contain toxic compounds.

Solid waste initially decomposes aerobically, with carbon dioxide the primary gas product. As oxygen in the landfill is expended, facultative and anaerobic microorganisms dominate. These bacteria continue to produce carbon dioxide, but the process moves into the anaerobic decomposition stage, where methane and carbon dioxide are produced in an approximate 50-50 ratio. Volatilization causes other compounds to be produced, and additional chemicals also are released into the landfill atmosphere [See “Landfill Gas Composition” on right].

Trace Constituents and Global Warming

In the past several years, there has been growing concern about the release of potential air pollutants from landfills. According to studies by the U.S. Environmental Protection Agency (EPA), Washington, D.C., landfills are the largest anthropogenic (man-made) source of methane emissions in the United States, accounting for 35 percent of the nation's total. Particularly, methane is a concern because it is considered a “greenhouse gas,” which contributes to global warming. (Landfills also are considered as carbon sinks because they hold carbon materials such as wood products, thus keeping the volatilized carbon in these materials from entering the atmosphere.)

Besides global warming concerns associated with methane generation, there are additional worries about landfills being sources of volatilized toxic metals, such as mercury. Mercury sources include discarded fluorescent lamps, mercury thermometers and batteries. Regulatory interest in assessing mercury emissions from landfills is growing.

Other pollutants are under investigation, too. In monitoring LFG for trace constituents, varying concentrations of higher molecular weight hydrocarbons have been found. Some of the compounds have raised concerns as toxic air pollutants.

In 1996, LFG emission concerns prompted the EPA to issue a regulation requiring landfills that emit LFG quantities in excess of approximately 50 tons per year to control emissions. Also, the regulations require that new and existing landfills designed to hold more than about 2.5 million tons of waste install gas collection systems or prove that the landfills emit less than 50 tons per year of non-methane organic compounds, including smog-causing volatile organic compounds (VOCs) and air toxics.

Many states now require landfill operators to monitor LFG and the air quality around the landfill perimeter.

On-Site Safety Practices

Methane gas concentrations in excess of 5 percent are explosive. LFG can asphyxiate a person who enters an enclosure containing gas. The following safety guidelines are recommended for landfill personnel:

Do not enter a vault or trench on a landfill without first checking for methane gas and/or wearing a safety harness. A second person should be standing by to pull the employee entering the landfill to safety, if necessary.

During landfill well installation, wear a safety rope to prevent falling into the borehole

Do not smoke during drilling or during installation of landfill gas wells or a collection pipe system. Also, do not smoke when LFG is venting.

Clear gas that is collected from a mechanically evacuated system to minimize air pollution and any potential explosion/fire hazard.

It is important that operators receive safety training and are trained to use monitoring equipment. Monitoring equipment and other devices should be routinely calibrated and maintained to ensure that accurate results are obtained.

Mechanics of Gas Movement

The mechanics of gas movement through refuse and soil are extremely complicated. Gas tends to migrate from the landfill through the refuse and surrounding soils on the path of least resistance. Gas will migrate further through sand and gravel soil than through silt or clay soil. The migration rate is strongly influenced by weather conditions: When barometric pressure is falling, gas tends to be forced out of the landfill into the surrounding soils formations. As barometric pressure rises, gas may be retained within the landfill for a short time period as new pressure balances are established.

Wet surface soil conditions and frozen ground may prevent gas from escaping into the atmosphere at the edge of the landfill. However, this could potentially cause the gas to migrate even further from the landfill. The maximum migration distance of methane gas is difficult to predict, but distances greater than 1,500 feet have been observed.

The Effect of Caps

Controlling gas movement at a landfill begins by studying local soils, geology and the surrounding area. For example, if the landfill is surrounded by a sand or gravel soil, and if buildings are close to the landfill, gas movement will need to be controlled by engineering methods.

On the other hand, a landfill surrounded by clay in an isolated location may not need as stringent a control system.

Nevertheless, care should be taken. The clay cap installed during landfill closure to exclude moisture infiltration and to restrict leachate generation tends to contain LFG. The pressure gradient that results will force the gas to move laterally and into the landfill's surrounding areas. Even a narrow sand seam in a clay formation can transmit large amounts of gas.

Probes are used to detect methane migration in the formations around a landfill [See “Typical Gas Probe” on page 48].

The probe is installed by boring a hole into the ground to at least the same depth as the landfill. A perforated pipe is placed into the hole and the space between the original soil, and the pipe is filled with sand. Clay is packed around the pipe near the ground surface to prevent air from leaking into the probe. Two types of measurements are conducted.

Gas pressure is measured with a gauge or manometer. A positive reading indicates that LFG is moving past the probe because of pressure built up within the landfill. Negative pressure typically results when a probe is installed near a LFG recovery well.

The concentration of methane in the soil atmosphere also is measured with a calibrated meter. A concentration greater than 5 percent methane indicates migration may have dangerous consequences if the gas enters a building.

Because migration patterns and methane concentrations change rapidly, frequent measurements are required to obtain an accurate picture of the gas migration pattern. At sites where a high degree of concern about gas migration endangering residences exists, daily measurements should be conducted until the crisis has passed.

At some sites, multi-level probes are installed to obtain a more accurate three-dimensional picture of gas movement.

Gas Vents and Recovery Systems

Passive vents and active gas pumping systems are used to control LFG migration. Passive systems rely on natural pressure and convection mechanisms to vent the gas into the atmosphere.

Shallow gas venting trenches, or gas venting pipes, installed within the landfill and vented into the atmosphere have been used to allow gas from a landfill's interior regions to escape. These natural vents may be equipped with flares to burn-off the gas and to prevent odors.

Passive vents do not always effectively remove LFG from under the cover. This causes vegetative stress and accompanying erosion problems on the cover. Passive vent failure generally is attributed to an insufficient pressure gradient within the landfill to push the gas to the venting device. Passive vents also can be problematic when alternating periods of high and low barometric pressure cause atmospheric air to enter the landfill when barometric pressure rises.

Passive systems are not considered reliable enough to be the sole means of protection in areas where there is a significant risk of methane accumulation in buildings. Active systems should be used in moderate- or high-risk areas.

Active Gas Recovery Systems

Active gas collection systems remove LFG under a vacuum from the landfill or the surrounding soil formation, with the gas literally being pumped out of the ground.

These systems may provide migration control or recover methane for energy recovery purposes. Both use gas recovery wells and vacuum pumps. A pipe network connects wells with the blower equipment.

When the primary purpose is migration control, recovery wells are constructed near the landfill's perimeter. Depending on site conditions, the wells may be placed in the waste or in the soil formation immediately adjacent to the landfill [See “Gas Well Placement” on page 52].

The location depends on site access, the soil formation around the site and the type of waste in the landfill.

At landfills where waste has been placed up to the property line, there may not be sufficient space to place the wells and collection lines outside the waste.

The surrounding soil formation must be evaluated before deciding where to place the wells. A sandy soil is more gas permeable than a clayey soil, and therefore more suitable for well installation. Some wastes may contain materials, such as large pieces of concrete, that prevent borehole drilling and recovery well installation. In extremely high-risk areas, wells may be installed in the soil formation before the waste is placed in the landfill so that the control system is operational before waste is deposited.

Collecting and Combusting LFG for Energy Generation

There has been a heightened interest in recovering energy from LFG, as landfills have grown larger and regulatory pressure to collect and combust gas has increased. Besides improving economies of scale by collecting gas from large landfills, some landfills can generate electricity and qualify for special “green energy” rates if they sell energy to public utilities. Energy from waste is classified as “renewable.”

Instead of burning and venting gas directly into the atmosphere, it is cleaned and directed to a combustion system, which uses the heat from burning the gas directly, or directs the gas to a turbine to produce electrical energy.

Especially where energy recovery is the goal and collection efficiency is paramount, gas recovery system construction is extremely important [See “Gas Recovery Well” on page 50].

In a gas recovery well, boreholes generally are 2 to 3 feet in diameter. Larger diameter holes provide more surface area at the refuse-gravel interface and require less suction for gas removal. This configuration is above ground to provide easy access to piping. An alternative design is to connect the well and header pipe entirely below ground. This is best suited for landfills where all equipment must be out of sight.

Well depths ranging from 50 percent to 90 percent of the refuse thickness are common, except where groundwater conditions are encountered. In this case, the well stops at the water table. The well casing usually is made from a slotted, plastic pipe.

It is important that wells be individually valved to regulate the vacuum that is applied to each well. Gas probes monitor the performance of the control wells.

If migration problems continue near a particular well, the control valves are adjusted to pump more gas with that well. Conversely, if methane measurements show low concentrations, the vacuum is reduced to draw in less air.

If gas is being recovered for its energy value, wells may be constructed along the landfill's perimeter for migration control, as well as placed in a grid pattern to recover gas that might otherwise escape through the landfill cover.

Before constructing an energy recovery system, tests usually are conducted to predict the quantity and quality of gas that may be available. Testing is important because wide variations have been observed in gas generation rates. Some landfills, depending on the waste types that may have been received, will have gases with chemical characteristics that require special handling. A pumping test is conducted by installing a gas recovery well and several monitoring probes in the landfill. The well is pumped until the gas flow stabilizes. Chemical characteristics of the gas are measured to determine the methane content and concentration of other chemicals. Concurrently, problems are monitored for pressure drop and methane content.

Using the results from one or more test wells, a full-scale gas recovery system can be designed. In addition to pumping test results, recovery system designs also should consider impermeable layers or walls within the landfill that may retard gas movement. The design also should examine regions within the landfill where liquids may fill the recovery wells.

Energy Recovery Method

The energy recovery method will primarily depend on the available energy markets.

If a factory or large building is near the landfill, it may be practical to pipe the gas directly into a boiler at the facility. The LFG can be passed through filters to remove moisture and possible hydrogen sulfide, and then injected into the furnace in combination with the regular boiler fuel, which may be coal, oil or natural gas.

Boiler fuel is perhaps the simplest approach to using LFG, but availability of a boiler near a landfill is not common. When deciding how far to transport the gas, the cost of constructing a pipeline between the site and the boiler must be compared with the gas's value.

If a boiler is not available, LFG can be directed to an engine-generator system for electricity production. Almost all landfills have electrical service, and the generated power can be put back into the electric grid [See “Typical Electrical Generation System” above].

In this case, the gas requires minimal treatment before being used as fuel in the gas turbine. A typical turbine generator system will produce 3.3 megawatts of electricity, consuming 1,600 standard cubic feet per minute of 500 Btus per cubic foot of LFG. Internal combustion engines also are being used to operate the generators.

Because the gas methane content will directly affect the turbine's performance, it is important that site operators closely regulate the gas collection system. Electricity generation from LFG is expected to increase greatly in the near future.

Other LFG use options include vehicle fuel. Experiments currently are being conducted on fuel cells.

As with any resource recovery project, the key to beneficial use of LFG is finding a good market for the recovered energy.

Phil O'Leary and Patrick Walsh are professors with the Solid and Hazardous Waste Center at the University of Wisconsin. Visitwww.wasteage.comfor more information.

Landfill Gas Composition

Methane

47.4%

Carbon dioxide

47.0%

Nitrogen

3.7%

Oxygen

0.8%

Aromatic-cyclic hydrocarbons

0.2%

Paraffin hydrocarbons

0.1%

Hydrogen

0.1%

Carbon monoxide

0.1%

Hydrogen sulfide

0.01%

Trace compounds

0.5%

Source: Ham, R., U.S. EPA,Recovery Processing and Utilization of Gas from Sanitary Landfills,1979